Poor input power factor and high input current total harmonic distortion (THD) generated by phase controlled and uncontrolled diode bridge rectifiers are well known problems in the power converter/rectifier industry today. Such low power factor and high THD commonly lead to input AC voltage distortions, distribution system losses, neutral harmonic currents and excitation of system resonances. To combat these problems, designers have attempted to develop improved three-phase rectifiers or converters which draw nearly sinusoidal line currents with low harmonic content and with high displacement power factor.
Three major power factor and harmonic current improvement approaches are commonly used in the industry. For medium to high power applications, AC/DC rectifiers (utility interface) often employ six-active-switch boost rectifiers. By using the proper modulation techniques for the six switches, it is possible to control the rectifier output voltage while maintaining nearly sinusoidal input line currents at unity power factor. However, this approach experiences limitations such as high system costs due to the need for a complex pulse-width modulation (PWM) control scheme for the six active power switches and associated drive circuitry.
Another approach for power factor and harmonic current improvement includes the use of three single-phase power factor correction (PFC) isolated AC/DC rectifiers. Such a configuration is more attractive than the previous approach because of a reduced number of active switches. The three single-phase PFC isolated AC/DC rectifiers, however, require three isolated high frequency DC/DC converters and, hence, it is still an expensive and complex PFC approach.
A single-switch, discontinuous conduction mode (DCM) three-phase boost PFC converter with six-diode bridge rectifier has been proposed and immediately drew interest due to its simplicity and low cost. This scheme illustrates that three-phase, low harmonic rectification is possible without either the use of large, low frequency passive elements or multiple active power switches and complex control. However, since such a topology and control scheme originates from a single-phase counterpart, it suffers from the same problems as its single-phase counterpart, namely, high switch peak-current stress, a requirement of fairly large electromagnetic interference (EMI) filters and a very high DC bus voltage for lowering the THD.
Although a sinusoidal peak current can be obtained by simply applying constant duty cycle control, the average input line current is not sinusoidal due to the non-proportional inductor discharging interval, which results in a relatively high input current THD. To lower the input current THD, the non-proportional discharging interval can be alleviated by increasing the DC bus voltage (i.e., increasing the voltage transfer ratio), which, in turn, increases the switch voltage stress and cost. In practice, designers have demonstrated that, even though problematic, the single-switch three-phase DCM boost converter is suitable for low cost, low power, medium performance three-phase AC/DC applications such as telecommunication applications due to its low cost, simple circuitry and simple control requirements.
Today, the power supply industry for telecommunication systems and the like has become very cost sensitive, with low production costs being a key to success. Additionally, the three-phase system THD requirement for telecommunication applications varies. Typically, the international market THD requirement for a three-phase AC/DC rectifier is often about 20%. While for the domestic market, the THD requirement for a three-phase rectifier system is often below 40%. Since the single-switch three-phase boost converter is operated in continuous conduction mode (CCM), the power switch stresses, DC bus voltage level, EMI filter size and rectifier costs are lessened. Therefore, a commercial three-phase rectifier can have a lower parts count at a reduced cost. Unfortunately, the drawback to such a power conversion approach is that relatively high THD (32%) and relatively low power factor (0.92-0.95) are experienced.
Accordingly, what is needed in the art is a cost effective three-phase power conversion topology that achieves an increased power factor and reduced input current THD.